You seriously overcooked it. You need to use something a little tougher in "low and slow" cooking so that when it breaks down it hits tender and wonderful when you're done. Prime rib needs fast, hot cooking and will be melt in your mouth delicious.

__________________

__________________You're only given a little spark of madness. You mustn't lose it. Robin Williams Alix

Prime Rib (or Standing Rib Roast) does not need a long cook-time. I cook mine low and slow in the oven at 325 until it is about 10 - 15 degrees under what I want. I then take it out, let it rest for 20 minutes, covered with foil. While resting kick the oven temp up to 450 degrees. After 20 minutes take off the foil and put the roast back in the oven to crisp up the fat. Let rest again for @ 10 - 15 minutes, slice and eat!!